The invention relates to cooled multiphase choke assemblies.
It is known to use an ‘output choke’ in connection with an inverter, such as an inverter of a frequency converter.
The output choke of a frequency converter limits the derivative du/dt of the output voltage of the converter and thus protects the device supplied by the frequency converter. If the device to be supplied is a motor, the output choke protects windings of the motor from partial discharges and restricts bearing currents in the motor, caused by common-mode voltage formed by pulse-shaped three-phase output voltage of the converter.
In high-current frequency converter assemblies it is known to connect switch components in parallel in order to achieve the required current strength. Thus, a frequency converter connection can comprise a plurality of output branches per each phase.
Published application WO 2004/019475 A1 “Output choke arrangement for inverter, and method in conjunction therewith” discloses an output choke assembly of an inverter, where a choke coil is provided for each branch of a phase of the inverter output. The publication discloses an assembly, in which each phase comprises three choke coils arranged symmetrically in a triangular shape, in which case the magnetic coupling between parallel branches of each phase is small and symmetrical. A structure, in which a choke coil is provided for each branch of the output, balances the currents of the switch components of the different output branches and facilitates the control of breakthroughs of the components.
The output choke assembly can be cooled in order to remove heat generated by the losses therein. It is known to position a cooling element inside a choke coil in such a manner that the flow of a coolant is guided into the choke coil from its first axial end and out of the choke coil from its other axial end. The coolant thus flows through the choke coil in the axial direction. The axial direction of the choke coil refers to a direction substantially parallel to the magnetic flux which is formed inside the choke during use.
The problem of cooled output choke assemblies is complexity. For each choke coil, there must be a cooling element with both an inlet connection and an outlet connection for the coolant. Consequently, in a three-phase inverter assembly with three output branches for each phase and one choke coil for each branch, there are eighteen coolant connections altogether. Such an assembly requires a lot of space and is complex and expensive to manufacture.